There has conventionally been known a focus detection method of detecting the focus state of a photographing lens from the phase difference between a pair of image signals whose relative positions change depending on the focus state of the photographing lens. This method is disclosed in, for example, Japanese Patent Laid-Open No. 5-127074. According to this method, a so-called pupil-dividing focus detection optical system forms a pair of object images on a photoelectric conversion element from a pair of light beams having passed through a photographing optical system. The photoelectric conversion element photoelectrically converts the object images into object image signals. The object image signals undergo predetermined calculation, calculating the defocus amount of the photographing optical system.
In the pupil-dividing focus detection optical system, the photoelectric conversion element includes the first focus detection pixel array having the first incident angle distribution and the second focus detection pixel array having the second incident angle distribution shifted from the first one. The direction in which the incident angle distribution shifts is called a pupil-dividing direction. By pupil division, a pair of focus detection pixel arrays receive light beams having passed through a pair of different photographing lens pupil regions shifted in the pupil-dividing direction. As the focus state of the photographing lens changes, a pair of optical images formed via a pair of pupil regions move in opposite directions along the pupil-dividing direction. By forming the first and second focus detection pixel arrays in the pupil-dividing direction, the moving amounts of a pair of optical images are detected to estimate the focus state. This is the principle of the pupil-dividing focus detection.
The pupil-dividing focus detection device sometimes suffers an eclipse of a light beam used for focus detection depending on a combination of the photographing optical system and focus detection optical system. A nonuniform eclipse adversely affects the focus detection precision, and in the worst case, focus detection fails.
Japanese Patent Laid-Open No. 3-214133 discloses a technique of reducing the adverse effect by the eclipse. According to Japanese Patent Laid-Open No. 3-214133, light quantity distribution information which associates a decrease in light quantity by the photographing optical system and focus detection optical system with a position on the surface of a light receiving element is calculated. Based on the light quantity distribution information, the focus detection range is defined or the signal is corrected.
Japanese Patent Laid-Open No. 3-214133 considers a decrease in light quantity, but does not consider the inclination of the barycenters of a pair of pupil regions of the focus detection optical system toward the pupil-dividing direction.
If a pair of pupil regions are extracted into a shape inclined in the pupil-dividing direction owing to the eclipse of the photographing lens, their barycenters incline in the pupil-dividing direction. The first and second focus detection pixel arrays are formed parallel to the pupil-dividing direction. For this reason, the direction in which a pair of object images formed via a pair of pupil regions move deviates from the direction of the first and second focus detection pixel arrays. If the image moving direction and detection pixel arrangement direction do not match each other, a parallax occurs between a pair of optical images detected by the first and second focus detection pixel arrays. The parallax changes the interval between detected images, generating a focus detection error.